1. Field of the Invention
The present invention is related to telecommunications network interconnection. More particularly, the present invention is related to ISDN-type networks.
2. Discussion of Related Art
Facsimile transmission of group G.sub.4, defined by CCITT Recommendation T 563, is effected by digital data transmission networks which are essentially of the integrated services digital network type (ISDN).
In France, ordinary access to the ISDN circuit, known as NUMERIS, is effected at a data transfer rate of 64,000 bits/sec (64 kilobits/sec).
In the United States, to take another example, access is effected at 56 kilobits/sec. At the interface of the French and American ISDN networks, the network of the transmitter effects a transfer rate conversion in accordance with CCITT Recommendation V110, namely:
from the United States towards France, one bit, set at 1, is added (padded) for every 7 bits; PA1 from France towards the United States, one bit out of 8 is removed.
The stream of bits is structured in frames in accordance with Recommendation V110 and the eighth bits, to be added or subtracted depending on the direction, are located at well-determined places within frames. In the case of a communication from France towards the United States, since the French network at the interconnection removes one bit out of 8, the French transmitting facsimile machine is so established that this subtracted bit is a useless bit and therefore in order to add a useless bit, set at 1, every 7 bits transmitted and therefore to reduce the effective transfer rate from 64 kb/sec to 56 kg/sec. In the opposite direction, from the United States towards France, the French receiving facsimile machine is adapted to eliminate one bit out of 8 bits received.
In the case of a strictly French communication, the facsimile machines obviously employ the total transfer rate of 64 kb/s.
In order to cause a facsimile machine to operate at full or reduced effective transfer rate, the user at present operates a two-position switch, one national (64 kb/s) and the other international (56 kb/s).
This method is unsatisfactory. First of all, the user either does not know, or does not wish to know, the nationalities of his correspondents and the technical characteristics to be associated with them. Secondly, and even if automatic processing of the outgoing calls were effected by the facsimile machine as the result of analysis of the country codes of the telephone numbers, this processing would not be infallible in view of the fact that a national network may develop or possess several data transfer rates. Thirdly, even if one wished to provide an automatic processing of the incoming calls, a call signal would be required which supplies the numbers of the calling parties, which is still not uniformly the case.
In the course of considering the effective transfer rate, of a G.sub.4 facsimile machine connected to an ISDN network, the applicant has addressed the more general problem of the operation of any facsimile terminal connected to an ISDN network. As will be seen below, the scope of the solution extends even beyond this application.
The problem having been raised, it is in fact by realizing that every G.sub.4 facsimile communication commences, in accordance with Recommendation T 563, by the exchange in the two directions of contiguous HDLC flags that the applicant has been able to devise the solution provided by the present invention.
Incidentally, after this exchange of flags and before the data frames, the calling party issues an SABM (set asynchronous balanced mode) frame which the party called acknowledges by a UA (unnumbered acknowledge).
An HDLC (high rate data link control) flag is a sequence, identical from one network to the other, of six 1's surrounded by 0's (01111110). In the hexadecimal system, a HDLC flag can be written 0111 1110, namely 7 E. The HDLC standard prevents the appearance of more than five consecutive 1's in the ordinary data, that is to say, in the data frames. For this purpose, a 0 is intentionally inserted after every sequence of five 1's.
If the flags come from a 64 kb/s network they are received, with an identical form, on a national reception terminal (French) which is also connected to a 64 kb/s network. If they come from a 56 kb/s network, they undergo modification at the interconnection of the two networks.
Let us consider, for instance, an initial stream at 56 kb/s: EQU . . . 1110011111100111111 . . .
At the interconnection, a 1 bit is added every 7 bits and the stream received at 64 kb/s has the following form: EQU . . . 1110101111111001111
In short, the flags received are no longer standardized: Regardless of the relationship between the framing of each communications starting flag and the inserted eighth bit "1", there appears rapidly, at the end of one or two flags, the hexadecimal sequence 7 F, which exists only in flags that are coming from a 56 kb/s network and received on a 64 kb/s network.
Having, by way of exception, substantially explained the the effect produced by the invention, it is now easy to formulate it.